The invention relates to a method for producing a connecting structure between two superconductors, in particular magnesium diboride superconductors, which comprise a superconducting core wire enclosed by normally conducting metal, and to a structure for connecting two superconductors, in particular two magnesium diboride superconductors, which comprise a superconducting core wire enclosed by normally conducting metal.
The use of superconductors allows energy-saving and particularly stable operation of magnets, for example magnetic resonance magnets, having short-circuited superconducting leads (so-called persistent mode). In this case, the energized superconducting magnet is short-circuited through a superconductor. The short-circuited magnet then forms its own current loop, in which the current can flow essentially without resistance. In this persistent mode, the current source can be disconnected from the magnet, so that energy-saving operation is possible. The advantage of persistent mode operation is an extremely high stability of the magnetic field, which cannot be achieved in this way even with the best current sources.
The short circuit is achieved by using a short-circuit switch, the so-called persistent switch. For this purpose, the conductor ends of the magnet coil are connected by a superconducting wire which can be brought into normal conduction by heating and then has a comparatively high resistance. When the persistent switch is in the normally conducting state, the current then flows from the current source through the superconducting coil which can be energized or de-energized in this state. Once the magnet has reached the desired field strength, it can be switched over into the persistent mode. To this end, the persistent switch is cooled and becomes superconductive, so that the magnet and the superconducting wire again form their own current loop.
What are essential for maximally long and stable operation of persistent mode are the resistance and inductance of the resulting current loop. The resistance of the contacts or connection points, by which the persistent switch is connected to the wire ends of the magnet, usually via a connecting structure, is in this case essential, so that one prerequisite for stable persistent mode operation is the possibility of producing a superconducting connection of the superconductor ends.
In other applications as well, it is often necessary to connect two or more superconductors with the lowest possible resistance. For example, magnet systems are known which comprise a plurality of individually wound coils, in particular 4 or 8 coils, which are to be connected via such contacts.
In the recent past, it has already been possible to show that superconducting connections can be produced in particular between magnesium diboride wires (MgB2 wires). The superconducting connecting structure may in this case be produced on the basis of magnesium diboride or on the basis of other superconductors, for example NbTi. The superconducting connecting structure should have a current-carrying capacity which is as high as possible, in order not to become the limiting element for current operation. In particular, the contact surfaces between the wire ends and the connecting structure must have a high connectivity and therefore be transmissive for high super-currents.
In order to produce such connecting structures, it has been proposed for example in U.S. Pat. No. 6,921,865 B2, in order to connect two superconductors, to arrange a magnesium diboride powder between the superconductors. Specifically, it is proposed to compress the already reacted magnesium diboride with a high pressure, in order to improve the bonding properties. It is also known additionally to sinter the magnesium diboride powder. These so-called ex-situ methods, however, disadvantageously generate only point contacts between the individual grains, the sintering additionally degrading the superconductor wire.
Recently, therefore, in an article by X. H. Lee et al., “High critical current joint of MgB2 tapes using Mg and B powder mixture as flux”, Supercond. Sci. Technol. 21 (2008) 025017, it has been proposed to use an in-situ method in which a magnesium and boron powder mixture is used, which is only reacted to magnesium diboride in situ by heating in a protective atmosphere.
Good results have been achieved in this case, but not for the range actually relevant e.g. in magnetic resonance devices, in which magnetic fields that are greater than 0.5 T or temperatures above 10 K occur. A possible reason for this is that the contact area is not sufficient. Furthermore, wire degradation takes place because of the prior mechanical processing of the wire ends and, in particular, also owing to the heat treatment during the in-situ reaction. There is probably a rather poor connection on the contact surface. Further problems which may arise are the inhomogeneous diffusion of copper or other elements in from the superconductor wire, so that the properties of the magnesium diboride in the contact are impaired, as well as other reasons.